Problem: If $8a + 2b + 3c = 2$ and $3x + 3y = 1$, what is $-21c - 9x - 56a - 14b - 9y$ ?
Explanation: $= -56a - 14b - 21c - 9x - 9y$ $= (-7) \cdot (8a + 2b + 3c) + (-3) \cdot (3x + 3y)$ $= (-7) \cdot (2) + (-3) \cdot (1)$ $= -14 - 3$ $= -17$